What firmware are you on and bb version?
Did you try to update or restore to stock firmware before with itunes?
If you did and you had the ipad bb installed you might need to flash it again using redsnow.
But Im assuming now, let us know more details about your phone.
Follow AJ's instruction and downgrade to 4.1 and see any different. A friend of my has a 3GS, 4.3.3, 06.15 baseband on it. The phone did not get signal at all. Either it is showing searching or 1 bar without carrier name. I upgrade his to 5.0.1, reflash ipad baseband, install ultrasn0w 1.2.5 and it did not work at all. It was shown "No service". First, I thought it was fried. So, I told him I will downgrade it to 4.1 and see what happen. If it did not work, then I will give it back to him. Once I downgraded down to 4.1, flashed ipad baseband, installed ultrasn0w. T-mobile show up with full bar.
I am not sure what Apple has change with their firmware or internal process, I have noticed the following;
1. Even the phone has 06.15 baseband on it, you still can restore it to official 5.0.1 firmware without any error. It will restore the phone to factory setting and come up with activation screen. However, when you put the original sim card it, it won't read the sim. It can't activate via phone. If you try to activate while connecting to Itune, it said no sim card installed (even sim is in the phone)
2. Downgrading to 4.1. Same thing, no error will appear (1015 appear previously) and the phone will come up to activation screen. However, when you try to activate with original sim, Itunes gives you a message that the sim inside the phone is not original sim and can't be activate.